Among photosensitive resin compositions, ones to which photolithography is applicable are called photoresists and widely used in semiconductors, MEMS/micromachine applications, and the like. In such applications, photolithography is performed through the successive steps of pattern exposing on a substrate and developing in a developer for selective removal of the exposed areas or the non-exposed areas. The photoresists have a positive type and a negative type. A positive type allows the exposed portions to be dissolved in a developer. In contrast, the negative type allows the exposed portions to be insolubilized. In an advanced technology field, electropackage applications and MEMS applications require not only the capability of forming a uniform spin coating film, but also a high aspect ratio, the vertical sidewall profile of a thick film, high tight adhesion to a substrate, and the like. The aspect ratio is calculated from (resist film thickness)/(pattern line width), indicating the performance of photolithography as one of the important properties.
According to the composition having the main component of a bisphenol A type novolac epoxy resin disclosed in Patent Literature 1 and Non Patent Literature 1, a photosensitive image having an extremely high resolution and a high aspect ratio, and a cured product of photosensitive resin can be formed. However, the produced cured resin product tends to be weak for a physical stress depending on application, occasionally resulting in cracking (crazing) during development or when internal stress is generated. Accordingly, the cured resin product may cause not only reduction in adhesion depending on the type of substrate to which the resin composition is applied, but also peeling of the cured product from a substrate in some cases. All the problems occur during cure shrinkage of the composition due to the stress accumulated in the cured resin product. Large cure shrinkage causes bending (warpage) of a substrate in many cases.
Further, it has been found that a substrate with the cured resin product formed thereon causes peeling of the cured resin product from the substrate when subjected to an examination for resistance to moisture and heat that is an accelerated durability test (at 80° C., 100%, 24 hours). Accordingly, use of the cured resin product in applications in an MEMS package, a semiconductor package, a component for forming microreactors, and the like causes inconvenience with poor durability.
Patent Literature 2 discloses a resin composition including an epoxy resin, a novolac resin, a cationic photopolymerization initiator, and a filler is disclosed. The literature describes that use of the resin composition increases the adhesion of a coating film with reduced moisture permeation. It has been found, however, that the disclosed composition lacks resolution as a photoresist, and high adhesion cannot be achieved without inclusion of a filler. The resin composition in Patent Literature 2 is, therefore, not suitable for use in an MEMS package, a semiconductor package, a microreactor and the like.